1. Field of the Invention
The invention relates to a solenoid valve that outputs hydraulic fluid on the basis of an exciting current supplied to a coil.
2. Description of Related Art
There is a conventional solenoid valve that includes a shaft-shaped spool valve housed in a tubular sleeve so as to be movable in its axial direction, and that is configured such that a flow passage of hydraulic fluid and a flow passage area are changed by moving the spool valve in the axial direction (see, for example, Japanese Patent Application Publication No. 2006-112514, (JP 2006-112514, A)).
The solenoid valve described in JP 2006-112514, A includes a solenoid portion, a spool valve, and a spring. The solenoid portion moves a plunger using a magnetic force of an electromagnetic coil. The spool valve is pushed in response to an operation of the solenoid portion and moves within a valve hole of the sleeve. The spring urges the spool valve toward the solenoid portion.
In recent years, reduction in size and weight of this kind of solenoid valve has been demanded. For example, if this kind of solenoid valve is applied to a hydraulic control apparatus in an automatic transmission of a vehicle, a plurality of solenoid valves need to be arranged in an engine room. Accordingly, if each solenoid valve is large, it may be difficult to arrange the solenoid valves because the solenoid valves may contact other accessories. Further, if each solenoid valve is heavy, an improvement in the fuel efficiency of the vehicle may be hindered.